Alternatively, you can simply change it at the same time as the fixer, or at the end of each session. The exact throughput is hard to measure as developer carry-over and local variables will have differing effects on the lifespan, but if you use pH strips for a while you will be able to work out your typical throughput.

Please bear in mind that we recommend using a water bath in place of a stop bath for the best archival speed and economy. The stop bath product is provided primarily for multi-user darkrooms where it can be useful to prolong the life of the fixer and maintain reliability.